The thermal conductivity of paramagnetic gases is known to change under the effect of magnetic fields. The molecules of a paramagnetic gas have a permanent magnetic torque, which is oriented in an external magnetic field. As a result, there is not only a change in susceptibility and hence an increase in the magnetic flux, but the possibility of transmitting heat energy to adjacent molecules by collisions is also reduced due to the orientation of the molecules. This causes a slight change in the thermal conductivity of the gas. This effect is also manifested in a mixture of paramagnetic and other gases. Since the change in the thermal conductivity of a gas mixture depends on the concentration of a paramagnetic gas contained therein, the percentage, i.e., the concentration of the paramagnetic gas can be inferred by determining the change in the thermal conductivity of the gas mixture. The paramagnetic gases include especially oxygen and nitrogen oxides.
A prior-art device for measuring the concentration of a paramagnetic gas, such as especially oxygen, appears from DE 100 37 380, A1, and is characterized by a modulatable magnetic field source with an air gap, a modulation source for sending a modulation signal to the magnetic field source, a measuring element for sending a measured heat flow signal, which is arranged at least partly within the air gap and is heated to a working temperature by a power source, and by a filter means connected to the measuring element for separating fluctuations from the measured heat flow signal on the basis of the modulation of the magnetic field, wherein the changing amplitude of the fluctuations is an indicator of the percentage of the paramagnetic gas in the gas sample based on the gas-specific change in the thermal conductivity. The measurement of the concentration of the paramagnetic gas, especially oxygen, is carried out in an air gap of the electrically modulatable magnetic system, which air gap is equipped with a measuring gas sample holder. A corresponding measuring gas sample holder is known, for example, from DE 102 51 130, A1. The measuring gas sample holder described there may be arranged, for example, in a measuring head described in DE 102 41 244, C1.
A measuring element is fastened in the prior-art measuring gas sample holder on a bottom plate and a duct plate is cut out for routing the gas in the area of the measuring element and around the measuring element. The measuring gas sample holder is sealed in the upward direction by a cover plate with at least two holes for the gas inlet and gas outlet. The gas is routed in the duct plate in parallel to the bottom plate, on which the essentially planar measuring element is placed. The measuring element is located at a spaced location from the bottom plate by means of spacers and also has a distance from the cover plate. Gas being passed horizontally by the measuring element can diffuse in this manner into the areas above and below the measuring element. Vortices may develop because of pressure fluctuations or rapid changes in the velocity of flow of the gas flowing through the measuring gas sample holder, and these vortices are likewise oriented horizontally, i.e., in parallel to the bottom plate due to the routing of the gas, so that uniform admission of gas to the measuring elements by diffusion is made difficult and the signal may fluctuate. The signal-to-noise ratio is thus impaired.